Evaluation of Deep Tunnel Sewer System in Mitigating Urban Flood Damage: Econometric Approach

Abstract Climate change is intensifying the risk of urban flooding, particularly in densely populated cities, such as Seoul in South Korea. Deep tunnel sewer systems (DTSS) have been developed as a primary infrastructural response to mitigate flood risks; however, empirical evidence of their actual effectiveness remains limited. This study evaluates the flood mitigation effect of South Korea’s first operational DTSS, located in Yangcheon District in Seoul. Using district-level panel data on flood damage and rainfall from 2010 to 2023, we apply difference-in-differences and synthetic difference-in-differences methods to evaluate the effectiveness of the DTSS in reducing flood damage. The results consistently showed that the DTSS substantially reduced monetarily quantified flood damage, with estimated reductions ranging from 97.6% to 99.2% across the specifications and methods. These findings provide empirical support for the cost-effectiveness of large-scale flood infrastructure investments, offering real-world validation that moves beyond simulation-based assessments.